1. Field of the Invention
This invention relates to a microprobe having an extremely small radius of curvature at the tip end portion which is used for a probe for measurement of muscle activity current, a probe for STM (Scanning Tunneling Microprobe), a probe for high density recording-reproduction device or a probe utilized for an encoder which performs positional information measurement in micropositional determination, dimensional measurement, distance measurement, speed measurement, etc.
2. Related Background Art
In recent years, the recording capacity of data in recording-reproduction device has become increasingly greater, and the size of recording unit has become smaller, while the recording density higher. For example, in digital audio disc, the size of the recording unit has become even about 1 .mu.m.sup.2. There is active development of memory materials in its background, and an inexpensive and high density recording medium by use of an organic thin film of an organic dye, a photopolymer, etc. is now appearing on the market.
On the other hand, a Scanning Tunneling Microscope (hereinafter abbreviated as STM) capable of observing directly the electron structure of the surface atoms of a conductor has been developed [G. Binnig et al, Helvetica Physica Acta. 55, 726 (1982)] and it has become possible to measure real space image with high resolution regardless of whether it is single crystal or amorphous, and there is also the advantage that observation is possible at low power without damaging the medium. Further, it can be also used for various materials through actuation in the atmospheric environment, and therefore a broad scope of applications are expected therefor.
The STM utilizes the phenomenon that tunnel current will flow when a probe of a metal (probe electrode) and an electroconductive substance are brought near to a distance of about 1 nm under application of a voltage therebetween. The current is very sensitive to the distance change between the two, and by scanning the probe so that the current or the average distance between the both may be maintained constant, surface information of the real space can be obtained. In this case, the resolving power in the plane direction is about 1 .ANG..
By applying the principle of STM, it is possible to perform high density recording and reproduction sufficiently on the atomic level (several .ANG.). As the method for recording and reproduction in this case, it has been proposed to perform recording by changing the surface state of an appropriate recording layer by use of a particle ray (electron beam, ion beam) or a high energy electromagnetic wave such as X-ray and an energy ray such as a visible ray, a UV-ray, etc. and effect reproduction by STM, or to perform recording and reproduction by means of STM by use of a thin film layer of a material having a memory effect for the switching characteristic of voltage and current as the recording layer, for example, a .pi.-electron-system organic compound or a chalcogenide as the recording layer.
For performing recording and reproduction on the molecular level, the recording density becomes higher as the radius of curvature of the probe tip end opposed to the recording layer is smaller. Therefore a probe is desired ideally which has a sharpness of as small as about an atom level.
For measuring the muscle activity current of a human body, since a single cell of a human body has a small size of about 2 .mu.m, the radius of curvature of the tip portion of microprobe is required to be smallest.
Further, an encoder is constituted of a standard scale having the information concerning position or angle and a detecting means for detecting the information concerning position or angle by moving relatively thereto. The encoders are classified into several types depending on the standard scale and the detecting means, such as the optical encoder, the magnetic encoder, the capacitance encoder, etc.
As the encoder having resolving power of atomic level, there is the parallel movement detecting device disclosed in Japanese Laid-open Patent Publication No. 62-209302 applying the basic principle of Scanning Tunneling Microscope disclosed in U.S. Pat. No. 4,343,993 which is capable of observing the information of a sample surface with atomic-level resolving power as already known.
In the prior art, such an encoder is provided with a standard scale concerning length and a probe provided in proximity to the scale, and has the function of encoding by signal processing of the information from the tunnel current flowing between the standard scale provided with a driving mechanism and a probe as the signal source.
The probe for detecting the tunnel current of the above encoder is required to have a small radius of curvature at the tip end in order to provide a high performance and high resolution encoder.
In the prior art, the probe having such a tip with a small radius of curvature has been prepared by use of mechanical polishing and electrolytic polishing. According to the mechanical polishing method, it is possible to prepare a probe having a fine tip portion with a radius of curvature of 5 to 10 .mu.m by cutting and polishing a wire of fibrous crystal (Pt, etc.) by means of a clock lathe. According to the electrolytic polishing method, a wire of 1 mm in diameter or less (W, etc.) is held vertical in the axis direction, dipped in an electrolyte to about 1 to 2 mm, and the wire is subjected to electrolytic polishing by application of a voltage between the wire and the opposed electrode in the electrolyte, whereby a probe having a fine tip of about 0.1 to 1 .mu.m radius of curvature can be prepared.
However, of the preparation methods of microprobe of the prior art as described above, the cutting method has the drawback that the probe will be soon bend because stress is applied on the probe, while the electrolytic polishing method, although a finer probe as compared with the cutting method can be prepared, has the drawback that is extremely difficult to prepare a fine probe with a radius of curvature on the order of atomic or molecular level.
It is also difficult according to the method for preparing microprobe of the prior art to prepare a microprobe of which the radius of curvature at the tip is on atomic or molecular level with high reproducibility.
The probe electrode prepared according to the above method, because it is fixed on a device by holding with a screw or by spring force, is weak in rigidity so far as the tip of the probe electrode is concerned, namely having the drawback of low natural frequency.
By use of the probe prepared by the method of the prior art, because the tunnel current draw-out electrode is apart from the detecting portion, it is susceptible to acoustic vibration, floor vibration, electric noise, whereby the amount of the information detected by the probe is decreased.
Further, for example, in the high density recording-reproduction device as mentioned above, since recording or reproduction of data is performed by scanning the XY stage while moving one probe, the movement distance of the probe becomes longer, whereby there is involved the drawback that the recording and reproduction speed becomes slow.
Thus, it has been desired to have a probe having a small radius of curvature of the tip.